GSM or CDMA? On CDMA phones it may be a bit more likely for calls to go into voicemail if the data connection is active. Since you say "lately", I am assuming it worked fine before. I use Chatter on a GSM (Cingular) phone, can call come through OK. I hate to suggest this, but the only way to know for sure is to do some troubleshooting through process of elimination - if I were you I would do a good backup, then hard reset my device and then try the browser first, see if calls come through. If they do, then load chatter and try again. Go one by one to see where th problem starts appearing.

If verichat is running always on in the data mode and your checking email with chatter mail every 15 mins it is likely you are gonna miss some calls due to an active data connection. When the arrows are green, data is flowing either up or down, a call gets bounced to voicemail. This is for a sprint phone for certain. I hear the GSM phones are a little better about how they handle incoming calls.